1. Field of the Invention
The invention is related to microelectromechanical systems (MEMS) devices.
2. Description of the Related Art
In general, a MEMS transducer converts energy between electrostatic and mechanical forms. MEMS transducers include both sensors that convert motion into electrical energy (accelerometers, pressure sensors, etc.) and actuators that convert electrical signals to motion (comb drive, micromirror devices, oscillators). MEMS devices using capacitive transducers are easy to manufacture and result in low noise and low power consumption sensors and/or actuators.
Capacitive sensing is based on detecting a change in capacitance of a capacitor. If a known voltage is applied across the capacitor, changes in current due to capacitive variations will appear in response to motion of one plate of the capacitor relative to another plate of the capacitor. Typically, fixed DC bias voltages are applied to the mass and electrodes of a MEMS device. Changes in the DC bias voltages can change an apparent acceleration of a mass of a MEMS device and can increase noise in a MEMS sensor system.
Similarly, capacitive actuation is based on variation in capacitance of the MEMS device. A DC operating point is established by applying a DC bias voltage across the capacitor and an AC signal changes the capacitance causing changes in force on a plate of the capacitor. Variations in the DC bias voltage applied across the capacitor can affect the force on the plate and thereby change the resulting mechanical energy of the MEMS device. Variations in the DC transduction bias voltage applied across the capacitor can affect frequency of oscillation of MEMS resonators.
Accordingly, techniques for stabilizing a DC bias voltage across a capacitor of a MEMS transducer are desired.